Staphylococcus aures infections. Testing the virulence determination, Index of Infections Potential (IIP). The work group includes Dr. David Henderson, members of his Epidemiology Section, Dene Zierdt and myself. In this extended study, true infections will be rigorously determined. For the published work, significant S. aureus isolates were designated from site of isolation, as wound, abscess, blood, and other sterile fluids. The present study will more validly categorize isolates. The number of isolates for a phage pattern will further determine whether an IIP value can be assigned, according to statistical criteria. Application of the IIP to a few phage patterns, for example 92/94/96/292//D-ll, showed a decline in IIP over a period of years, after entry to our hospital wards. This goes along with the understanding that extraordinarily virulent S. aureus strains on entry, lose that quality over a period of years.